The demand for xylenes, particularly paraxylene, has increased in proportion to the increase in demand for polyester fibers and film. Processes for producing xylenes include transalkylation, disproportionation, toluene alkylation with methanol, among others, certain of which are capable of producing paraxylene selectively, i.e., in amounts greater than thermodynamic equilibrium (23 mol % based on the xylene isomers at typical processing conditions). Another important reaction in making paraxylene is the well-known xylenes loop, which involves the extraction of paraxylene, typically by adsorption or crystallization, leaving behind a paraxylene-depleted stream (“raffinate”), which is isomerized by liquid or vapor phase isomerization, or a combination thereof, to an equilibrium mixture of xylenes, followed by recycle to the paraxylene extraction step. All of these and other processes may be integrated in various ways, generally with the goal of optimizing paraxylene production economically. Virtually all of these processes, as well as many processes involved with the production of chemicals other than xylenes, utilize catalysts which benefit in some way by having their activity attenuated, at some point in the process, by sulfiding, coking, silicon-selectivation, and the like.
For instance, the manufacture of xylene using transalkylation processes may utilize one or more catalysts to convert feed streams containing benzene and/or toluene (collectively, C7− aromatic hydrocarbons) and feed streams containing heavy aromatics, i.e., C9+ aromatic hydrocarbons, into a xylene-containing product stream. See, for instance, U.S. Pat. Nos. 5,030,787; 5,763,720; 5,942,651; 6,893,624; 7,148,391; 7,439,204; 7,553,791; 7,663,010; 8,071,828; 8,163,966; 8,183,424; 8,586,809; and 8,822,363; Publication Nos.; 2013-0267748; 2015-0025283; WO2012/074613; WO2014/193563 and WO2012/173755.
A typical transalkylation process may comprise contacting the combined C7− aromatic hydrocarbon stream with the C9+ aromatic hydrocarbon stream with a first catalyst comprising a zeolite (e.g., ZSM-12, ZSM-11, and the like) and a hydrogenation component, such as a platinum-group metal, to provide for dealkylation/transalkylation, to produce a first product, and then contact of the first product with a second catalyst (e.g., ZSM-5), without a hydrogenation component, to crack certain undesired co-boilers, including those produced in the dealkylation process. Co-boilers are those species which boil at or near the boiling point of one of the desired aromatic products, making separation by fractionation difficult.
One of the key undesirable side reactions in such a transalkylation process, or any of the aromatics processes using a catalyst having a hydrogenation component, is ring saturation of the aromatic moiety, e.g., the aromatic ring is saturated to a naphthene, and the naphthene is then subsequently hydrocracked to lighter paraffins, namely C2 and C3 species. This has two impacts—it downgrades aromatics to fuel gas—and it may result in a higher amount of co-boilers.
On fresh start-up (the first time the catalyst contacts hydrocarbon feed) the hydrogenation metal, such as platinum, will ring saturate and crack at very high levels. This reaction is exothermic, and the exotherm can actually exceed the design temperatures of the equipment. Because of this, there is a need to temper the metal activity. One way to do this is to passivate the metal, i.e., lower the activity, to allow for start-up. One method of passivation is to use sulfur which, without wishing to be bound by theory, sorbs onto the hydrogenation metal and decreases its ability to cause ring saturation. For reactions such as transalkylation, passivation may include pre-sulfiding (before feed introduction) and/or co-sulfiding (meaning the sulfur is introduced with the hydrocarbon feed) the catalyst.
Sulfiding a supported metal catalyst by pre-sulfiding or co-sulfiding is a well-known technique but with several known negative effects, which include: (1) sulfur in the feed can result in sulfur in the product and/or potential sulfur poisoning of processes downstream; and (2) sulfur may cause permanent deactivation of some active sites, affecting the useful life of the catalyst. Thus, sulfiding is usually limited to pre-sulfiding or co-sulfiding for some short period of time during catalyst start-up.
In an example of a known process, U.S. Pat. No. 5,763,720 proposes pre-sulfiding a zeolite transalkylation catalyst system and subsequently co-sulfiding with introduction of a hydrocarbon feed stream at a sulfur concentration of 50 to 10,000 ppmw for up to 10 days. Additionally, U.S. Pat. No. 8,242,322 proposes intermittently introducing sulfur to a transalkylation catalyst after 2 days on stream in small quantities, i.e., 1-150 ppm by weight, in order to improve benzene purity in addition to or alternatively to in situ sulfiding within the first 2 days on stream.
After initial sulfiding, the catalyst can still be over-active. Because of this, the reactor is typically run at sub-optimal conditions during a “line-out” or “de-edging” period. For example, during de-edging, the reactor may be run with a partial pressure of hydrogen substantially less than designed in order to lessen ring saturation and cracking and controllably coke the hydrogenation metal of the catalyst. This de-edging period may typically last from several weeks to several months for transalkylation.
It has surprisingly been discovered that the de-edging period for transalkylation can be accelerated, i.e., optimal reaction conditions can be reached quicker, with no substantial deleterious effects on catalyst performance by pre-sulfiding/co-sulfiding according to a profile tailored to the exotherm of the transalkylation reaction.